Untreated metal surfaces are subject to corrosion which can lead to rust development, weakening, discoloration and failure of the surface. Thus metal substrates are typically treated by a variety of methods to make the surface less reactive and more corrosion resistant. In addition, metal surfaces are often subsequently coated with decorative or additional protective coatings such as resin coatings, primers, paints and other surface treatments.
One common mechanism of corrosion occurs galvanically when atmospheric oxygen oxidizes the metal of a metal substrate. The transfer of electrons from the metal to the oxygen at electrocatalytic sites leads to formation of various metal oxide corrosion products. A variety of treatments can be used to prevent corrosion of metal surfaces such as cold rolled steel, hot rolled steel, zinc, aluminum, and alloys thereof, zinc- and zinc alloy-coated steel, as well as aluminum- and aluminum alloy-coated steel. These treatments include temporary oil-based rust preventatives, phosphate conversion coatings, inorganic and organic passivates, paints, and combinations thereof.
Temporary oil-based rust preventatives are used to provide short-term corrosion protection with ease of removal. They are not desirable for medium- or long-term corrosion protection alone, and when combined with other coatings make the surfaces unpaintable and by the nature of their handling are not suitable for a final consumer product.
Phosphate conversion coatings provide better corrosion protection and paintability, but require quite acidic conditions to operate, which is not desirable due to difficulty in handling the application chemicals, more troublesome waste treatment due to sludge generation, and extra wear on equipment. Such conversion coatings typically require subsequent treatments for maximum benefit.
Inorganic and organic passivates, such as those commonly applied to metal coil stock, provide a high degree of corrosion protection but possess several drawbacks. Many such products either contain chromium, are highly acidic, or both. Chromium-containing coatings and passivates are not desirable due to negative environmental considerations and more troublesome waste treatment. Highly acidic treatments also tend to degrade processing equipment and pose concerns with worker exposure.
Another issue for coatings is the development of yellowing or a yellow hue in the coating after heat treatment of a coated substrate or other common treatments including exposure to alkaline cleaners, phosphating solutions or solvents.
It is highly desirable to create compositions that are effective corrosion protective coatings without these negative issues. Applicants have developed a sol-gel coating composition that reduces or eliminates one or more of the foregoing drawbacks.